1. Field of the Invention
Aspects of the present invention relate to a direct methanol fuel cell (DMFC).
2. Description of the Related Art
In general, fuel cells are electricity generators that convert the chemical energy of a fuel into electrical energy through a chemical reaction and can keep generating electricity as long as the fuel is supplied thereto. Among fuel cells, a direct methanol fuel cell (DMFC) generates electricity by supplying an anode with methanol and reacting protons from the methanol with oxygen supplied to a cathode. A DMFC usually has a cell structure as illustrated in FIG. 1.
Referring to FIG. 1, an anode 11 and a cathode 13 are disposed facing each other, and an electrolyte membrane 12 is disposed between the anode 11 and cathode 13. The cathode 13 is exposed to external air, which is used as an oxygen source. The anode 11 is surrounded by a spacer 14, and vaporized methanol is supplied to the anode 11 through supply holes 14a formed in the spacer 14. In the anode 11, a reaction as defined by Formula 1 below occurs, and thus, electrons are generated. The electrons move to the cathode 13 along a path 15 and participate in a reaction defined by Formula 2 below.CH3OH+H2OCO2+6H++6e−  <Formula 1>3/2O2+6H++6e−3H2O  <Formula 2>
As such, work can be done by using the generated electrons, when a load 16 is disposed in the path 15. An assembly of the anode 11, the cathode 13, and the electrolyte membrane 12 is conventionally called a membrane electrode assembly (MEA) 10.
Methods of supplying methanol to the anode 11 include a method of supplying methanol in a liquid state using a pump, and a method of guiding vaporized methanol to the anode 11, using the natural vaporization of methanol at room temperature. The latter method is called a passive method, and the associated structure is referred to as a passive DMFC.
When supplying the vaporized methanol to the anode 11, through the supply holes 14a of the spacer 14, it is difficult to control the amount of supplied vaporized methanol. After starting to supply the vaporized methanol, the vaporized methanol continues to flow through the supply holes 14a of the spacer 14. If the amount of supplied vaporized methanol is excessive, a combustion reaction of methanol, due to a cross-over/penetration phenomenon (crossover) of the vaporized methanol into the cathode 13, frequently occurs, and the DMFC can become overheated. Hence, if the methanol floods the MEA, the crossover can occur. Crossover generally refers to when methanol penetrates the electrolyte membrane 12 and crosses over to the cathode 13. The methanol crosses over to the cathode 13 and reacts with oxygen and combusts, and thereby sharply increases the temperature of the DMFC.
Conventionally, the spacer 14 is relatively thick, in order to increase a distance D between the exits of the supply holes 14a and the anode 11, to prevent methanol from flooding the anode 11. Therefore, the excessive supply of fuel to the anode 11 is prevented, because the concentration of the vaporized methanol gradually decreases, as the distance from the exits of the supply holes 14a to the anode 11 increases.
However, the thickness of the spacer 14 increases the volume of a unit cell of a DMFC and increases the total size the DMFC. Hence, according to the conventional structure, the distance D is increased, in order to prevent flooding, resulting in the DMFC having a relatively large size.
A device that can appropriately control the excessive fuel supply, without employing a thick spacer, would be beneficial in reducing the overall size of a DMFC.